Air strikes by the Syrian government and allied Russian forces left at least a dozen civilians dead in northwest Syria, where heightened bombardment has claimed hundreds of lives since late April, an activist war monitor said on Monday.

The bombardment also wounded 27 other civilians in total, the UK-based Syrian Observatory for Human Rights said.

Syrian government forces have since last week been chipping away at the opposition-dominated region of Idlib on the Turkish border.

This comes despite a deal reached between rebel-backer Turkey and government ally Russia last year to set up a buffer zone in the region to protect its three million residents.

Since Wednesday, the Syrian military has seized the key town of Khan Sheikhoun from Hayat Tahrir al-Sham - an alliance led by Syria's former al-Qaeda affiliate - and allied rebels, overrunning the countryside to the south and encircling a Turkish observation post in northern Hama, AFP news agency reported.

A new push by Syrian government and Russian forces to take the area has seen heavy strikes and advances this week, particularly south of Idlib province and nearby Hama. The advancing forces have caused a new civilian exodus, Reuters reported.

13 innocent civilians lost their lives yesterday August 25th, and 12 others were injured, due to the continuous shelling by the regime and Russian murderous offensive on #Idlib countryside. While death planes continue today, causing more civilian casualties. pic.twitter.com/acuBAlwPN8